Foamed clay process

ABSTRACT

Disclosed is a foamed ceramic material having a relatively small and uniform cell structure. The material is suitable for structural or insulating purposes. A continuous process for producing the foamed ceramic material comprises dropping a bloatable ceramic composition, in particulate form, through a heated zone thereby fusing and bloating the particles, and subsequently collecting the fused particles. To produce the desired cellular structure, the process does not fully bloat the particles during dropping, thereby delaying part of the bloating of the particles until after they have been collected as an agglomerated slab.

United States atent rubaker et al.

1541 FOAMED CLAY PROCESS [72] Inventors: Burton 1). Brubaker; NathanWaldman,

both of Midland, Mich.

[73] Assignee: The Dow Chemical Company, Midland,

Mich.

[221 Filed: Nov. 26, 1969 [21] Appl. N0.: 880,013

52 us. c1. ..264/43, 106/40 R, 161/159,

264/42, 264/55, 264/66, 264/125 51 Int. Cl. ..C04b 21/00, B28b 1/50 581Field 615661611 ..252/62, 378 R, 378 P; 264/42,

264/55, 121, 125, DIG. 21, DIG. 39, 43, 66; 106/40, 41, 72; 161/165,192,159

[56] References Cited UNlTED STATES PATENTS 3,056,184 10/1962 Blaha..264/42 X 1,877,138 9/1932 Lee et al. ..106/40 R 2,676,892 4/1954McLaughlin ....106/40 R X 2,706,844 4/1955 Nicholson ..106/40 R X June27, 1972 3,418,403 12/1968 Garnero ..252/378 R X 3,515,624 6/1970Garnero ...252/378 R X 2,271,845 2/1942 Parsons ..106/40 R 3,274,3099/1966 Schreieck ..264/42 Primary Examiner-Robert F. Burnett AssistantExaminer-Joseph C. Gil

AttorneyGriswold & Burdick, William R. Norris and Lloyd S. Jowanovitz 57ABSTRACT Disclosed is a foamed ceramic material having a relativelysmall and uniform cell structure. The material is suitable forstructural or insulating purposes. A continuous process for producingthe foamed ceramic material comprises dropping a bloatable ceramiccomposition, in particulate form, through a heated zone thereby fusingand bloating the particles, and subsequently collecting the fusedparticles. To produce the desired cellular structure, the processdoesnot fully bloat the particles during dropping, thereby delaying part ofthe bloating of the particles until after they have been collected as anagglomerated slab.

9 Claims, 3 Drawing Figures Comba-s I /onyases PATENTEDJUNN i9723,673,290

4 x I Q 12 14 15 r Comus/l'on &3 5 29 X,

annea/er INVENTORS. Z5 Bur/on 0 Bra/baker 26a 17/ M0 on Wa/aman Z9 1 W1M flTTOR/VEKS FOAMED CLAY PRocEss BACKGROUND OF THE INVENTION A numberof different processes have been devised whereby particles of abloatable composition (such as glass batches, and natural or syntheticclays) are treated in a heated zone to produce useful articles. Toillustrate, Christensen et al., (U.S. Pat. No. 2,151,083) droppedparticles of a clay composition through a heated zone to partially bloatthe particles into multicellular bodies. The bloated particles weresubsequently chilled and collected to produce lightweight aggregate.McLaughlin (U.S. Pat. No. 2,676,892) varied the process of Christensenby heating the particles to a higher degree before collection. Theresulting product was lightweight aggregate in the form of hollowunicellular bodies. A more recent process (similar to McLaughlin) formaking bloated aggregate is described by P. Weber et al., (U.S. Pat. No.3,409,450).

It has long been known that processes similar to McLaughlin orChristensen could be employed to make agglomerated products from clays.For example, Parsons (US. Pat. No. 2,271,845) dropped particles of claythrough a zone heated sufficiently to melt only the surfaces of theparticles. The particles were collected before cooling to form anagglomerated material.

The density and heat conductivity of agglomerated products such as thoseof Parsons can be reduced by bloating the clay particles with furtherheating after agglomeration. Further heating, however, raises problemsbecause the particles on the surface of the agglomerate tend to bloatfirst and form an insulating layer, which decreases the amount of heatreaching the interior of the agglomerate. The result is an agglomeratedmaterial of varying cross-sectional density and cell size.

Lee (U.S. Pat. No. 1,877,137) solved the insulating problem by formingthe agglomerate in a series of layers so that interior portions were onthe surface of the agglomerate sufficiently long to be adequatelybloated before they were covered with successive layers of particles. Toachieve this result, Lee employed a plurality of horizontallydisposeddropping zones to apply particles to a surface moving under the zones.Slidell et al., (U.S. Reissue Pat. No. 18,844) replaced the multipledropping zones and continuously moving surface of Lee with areciprocally moving inclined plane.

Subsequently, it was discovered by Blaha (U.S. Pat. No. 3,056,184) thatan agglomerate of uniform density could be prepared on a continuousbasis (as opposed to a batch-type basis) by bloating particles intoindividual hollow spheres of unicellular or multicellular form duringtheir passage downward through the heated dropping zone. The bloatedparticles, as collected below the dropping zone, are sufficiently hot sothat they coalesce during collection without application of further heatto form a lightweight, agglomerated structure, which possesses largecells and a density of at least 18 pounds per cubic foot (p.c.f.)

Agglomerated products similar to the Blaha product can also be producedby mold-bloating techniques. For example, Nicholson (U.S. Pat. No.2,706,844) using mold-bloating techniques, produced an agglomeratehaving a density as low as 20 p.c.f. Kitaigorodskii et al., (TrudyMoskov. Khim.-Tekh. Inst. im. D. l. Mendeleeva Pat. No. 24, 318,323(1957) describes mold-bloating experiments wherein agglomerates with adensity of 12.5 p.c.f. were produced.

A principal object of the invention is to provide a novel continuouspellet-dropping process for producing foamed ceramic materials.

It is an object of the present invention to provide novel agglomeratedfoamed ceramic materials having a uniform density of less than 12 p.c.f.and small (e.g., 2 mm) cells which are generally uniformly distributedthroughout the product.

It is another object of the invention to provide a foamed ceramicmaterial having low thermal conductivity.

Still another object of the invention is to provide a novel foamedceramic material having a density of less than about p.c.f. and as lowas 6 p.c.f.

DESCRIPTION OF DRAWINGS FIG. 1 is a schematic cross-sectional view of afurnace which can be employed in producing the foamed ceramic materialof the present invention.

FIG. 2 is a cross-sectional view of a foamed ceramic product beforemachining.

FIG. 3 is a cross-sectional view of the ceramic product after machining.

SUMMARY OF THE INVENTION In the present invention, individual particlesof a bloatable ceramic composition are passed downwardly through aheated dropping zone. The zone temperature is in excess of the initialmelting temperature of the composition and is sufficient to bloat theparticles to the point where each particle contains a plurality ofhollow cells. The bloated particles are collected below the droppingzone in a heated collection zone. The temperature of the heatedcollection zone and time of residence of the particles therein aresufficient to cause the collected particles to further bloat andcoalesce to form a foamed ceramic slab wherein individual bloatedparticles are not visually distinguishable. The slab is subsequentlycooled in a controlled manner (i.e., annealed) to minimize the effect ofthermal stresses which arise during cooling.

It is important in carrying out the invention that during the passagedownward through the heated zone, the particles are not bloated to fullcapacity, i.e., the particles should be bloated only to from about 50 toabout percent of capacity. The maximum bloating capacity (minimumdensity) of the particles is determined by a series of runs in whichaliquots of the foamable ceramic are dropped through the heated zone atincrementally increased temperatures. A plot of the bulk densities (asordinate) of the resulting bloated ceramic particles againsttemperatures (as abscissa) yields a cup-shaped curve, the minimum ofwhich defines the particle density at maximum bloating capacity.Particles bloated within the above range will retain a vestige of theiroriginal shape. For example, substantially all of the particles will benon-spherical in shape, assuming that the particles were produced byextrusion or slinging and were therefore non-spherical before they weredropped. A slinging process and apparatus is described in U.S. Pat. No.3,259,171, which is incorporated herein by reference as one meanssuitable for the preparation of particles.

It has been found that if the particles have fused and bloated to hollowessentially unicellular spheres, the point of minimum density will havebeen passed and the particle density will have started to increase abovethe minimum point. Thus, further expansion after agglomeration isunlikely. By not utilizing the full bloating capacity of the particlesduring dropping as in the practice of the instant invention, theparticles can be bloated further after they are collected. The gaseswhich normally escape as the particles are bloated to maximum potentialare trapped within the mass of collected parti- V cles to produce thedesired low density foams.

During the collection in the heated collection zone, the particles aresimultaneously bloated further and are coalesced to form the ceramicfoam product of the invention. Where simultaneous further bloating andcoalescence occurs in the collection zone, the zone is heated to atemperature from about 75 C. to about 200 C. below the temperature ofthe dropping zone.

Heating of the collection zone can be accomplished by positioningburners in close proximity to the zone, or by passing the heatedcombustion gases from the dropping zone through the collection zone.Preferably, both means of heating will be employed for control purposes.For efficient heating and temperature control in the collection zone, itis desirable that the zone be essentially isolated from the atmosphereoutside the furnace, and from other portions of the furnace exceptingthe continuous dropping zone.

In the above description of the invention, the functions of thecollection zone have been to complete bloating of the partially bloatedparticles received from the dropping zone, and

to cause coalescence of the particles. Presently, this is the preferredmethod for carrying out the invention. It should be understood, however,that the collection functions (i.e., further bloating and coalescence)need not be conducted simultaneously but could in fact be shared betweena number of zones. To illustrate, partially bloated particles werecollected in a heated zone wherein they adhered together and werefurther bloated to form an initial slab of ceramic materia]. Somecoalescence of the particles also occurred in the collection zone.Subsequently, the slab was passed into a third heated zone (the heateddropping and collection zones being the first two heated zones), wherebloating and coalescence of the particles were completed.

After coalescence has occurred, the slab is cooled as further exposureto the high temperatures of the collection zone may cause an undesirableincrease in cell size.

For purposes of the present application, the term melting indicates aheat-produced condition of particles of bloatable composition wherebythe interior and surface portions of particles become thermally plasticand capable of deformation without fracture by internally generated gaspressure. The mineralogical or physical components (e.g., glass-claymixtures, pure clay minerals, mixtures of clay minerals, etc.) of theceramic composition become at least partially chemically blended.Coalescence or coalesce indicates that collected particles unite ormerge into a single viscous body capable of trapping gases released fromthe particles. During coalescence, the individual particles lose theiridentity so that, on visual observation of a cross-sectional area of thefoamed ceramic material formed from the coalesced particles, a largenumber of small substantially uniform cells is seen. Very few, if any,vestiges of individual particles would be seen.

Because melting can take place very sharply, that is essentially at aconstant temperature, as for many pure crystalline chemical compounds oreutectic compositions, and also over widely variable temperature ranges,as for a complex mixture or the softening of glass, it is necessary todiscuss the determination and definition of the initial meltingtemperature" of the bloatable ceramic compositions. In determininginitial melting temperature, the ceramic composition is thoroughly mixedand ground by ball milling to pass a 200 mesh sieve. The powder istamped lightly into a sample holder for differential thermal analysis(DTA). A DTA test is run using a heating rate of about C. per minute.The DTA test is carried out in a gaseous atmosphere of helium flowing ata rate of 0.4 liter per minute. The DTA apparatus employed is a DTAModule, Model Number DTA-Al, manufactured by the Harrop PrecisionFurnace Company. The sample holder, consisting essentially of nickelprocessed the same dimensions as the standard beryllium oxide holder,supplied with Model Number DTA-All. The reference material was ignitedalumina. The thermocouples were a platinum versus platinum-lO-percentrhodium.

Melting is an endothermic reaction and will be recorded as anendothermic peak on the DTA record. The first DTA test is run to atleast 1,000 C. and until a strong endothermic reaction has begun. Astrong endothermic reaction is one which gives a recorder peak of atleast one-fifth full scale deflection from the base line on a DTAinstrument calibrated so that a reagent grade precipitated calciumcarbonate (e.g., calcite) gives a decomposition peak maximum of at leastfourfifths full scale.

Where a strong endothermic reaction is observed, heating is stopped andthe sample is cooled. Whether the endothermal peak is due to the onsetof melting is determined by physical examination. If melting has begun,visual observation with a light microscope or a scanning electionmicroscope will reveal some bonding together of the particles of powderand occasional fracturing of particles which have bonded. Also, thepresence of small spheres in the sample and rounding of the particleswill be observed, especially along surfaces created by fracturing thesample, as in removing it from the DTA cup. The physical integrity ofthe sample will progress from a point where before heating began it waseasily pulven'zed between the fingers, until after initial melting hasbegun, when pulverization with the fingers is difficult and incomplete,i.e., the sample has become hard and agglomerated. If melting has beensufficient to allow bloating and expansion, the sample will probablyhave to be chipped out of the sample holder, and the initial meltingtemperature obviously has been exceeded.

Where the above procedure is followed, a subsequent DTA test can be runto a termination point of l0-20 C. below the temperature at which theendothermal peak under investigation began. The sample, when cooled,should be either a loose powder or slightly compacted, but still easilyreducible to a powder by rubbing between the fingers. None of therounding of particles, or bonding of particles and fracture of particlesshould be observable with a microscope.

The initial melting point is defined as the temperature at the beginningof the strong endothermic peak (on a DTA curve) which has beenidentified as melting by the visual observation techniques describedabove.

The invention can be better understood by reference to FIG. ll. Thefurnace (indicated by reference numeral 1 1) consists of a pelletdistribution device (indicated by reference numeral 13) consisting of apellet storage bin casing 10, which defines a storage chamber 8containing pellets 9 of a bloatable ceramic composition. Casing l0 restsupon feeder housing 4 which defines a passageway 12 extending fromstorage chamber 8 to chamber 14 wherein a horizontal grooved drum 15 isrotatably mounted on hollow shaft 16. The grooved drum is in meshingengagement with cleaning wheel 18. The feeder housing 4 defines aconduit 20 leading to a dropping zone 21. Conduit 20 connects the pelletdistribution device with the dropping zone 21.

In the walls of the vertical dropping zone, housing 19 defining droppingzone 21 are housed a plurality of recuperative burners 22. The burnersare supplied with a combustible mixture of gas and air. Heat is directedtoward the center of the furnace.

The dropping zone housing 19 discharges heated particles and heatedcombustion products into collection and cooling zone housing 27. Thishousing defines a cavity 24 wherein a pellet collection apparatus(indicated generally by reference numeral 25) is mounted. The pelletcollection apparatus 25 consists rotatably mounted cylinders 26 and 26awhich support a product collection belt 32. Positioned above the belt 32are burners 29. Positioned below the belt are burners 28. The portion ofcavity 24 immediately below the dropping zone 21 is collection zone 33wherein pellets 17 are collected to form a slab of foamed ceramicmaterial 40. The slab is transported by belt 32 through a controlledcooling zone 34 adapted with burners to allow for gradual cooling athigh temperatures. From the cooling zone 34, the foamed ceramic may befurther processed in an annealer which provides for gradual temperaturereductions to minimize internal stresses (not shown). A plurality ofproduct removing rollers 41 are mounted in the controlled cooling zone34. Apparatus 42 for distributing parting agent onto the belt 32 ispositioned at the end of cavity 24 opposite the annealer.

To practice the invention, pellets 9 of a bloatable ceramic compositionare placed in chamber 8 and pass through conduit 12 to contact grooveddrum 14. As drum 14 is rotated, the particles thereon are dislodged bythe force of gravity and pass downwardly through conduit 20 and droppingzone 21. Any particles not dislodged from drum 14 by gravity are crushedby wheel 18 and are thereby removed from drum 14.

Dropping zone 21 is heated by burners 22. Due to the essentially closednature of the furnace, hot products of combustion pass downwardlythrough the dropping zone to the collection zone. In passing downwardlythrough the furnace chamber, the combustion products create a gascurrent which causes the ceramic particles to fall at a rate slightlyfaster than that produced by gravitational forces alone. As theparticles 17 pass through the dropping zone, they are bloated until eachparticle contains a plurality of hollow cells as hereinbeforeprescribed. By the time the pellets have reached the collection zone 33,they will have obtained a temperature within the range of from about 800C. to about 1,700 C. The multicellular particles are collected in zone33 on belt 32. As the belt turns, it is coated with a parting agent suchas sand or expanded clay. The parting agent prevents the fused ceramicfrom reacting with, or adhering to, the collection belt. It also servesto insulate the belt from the high temperatures of the collection zonethereby giving the belt longer life and allowing for the use of highertemperatures in the collection zone than could normally be employed. Theparting agent generally is (but need not be) sufficiently refractory soas not to be selfbonding at the temperature of the collection zone.

As the particles are collected, they bloat further and coalesce to forma slab 40 of foamed ceramic material which is passed through the coolingzone 34 and conveyed on rollers 41 to the annealer (not shown). Bloatingand coalescence are substantially accomplished in the collection zone.

The purpose of cooling zone 34 is to cool the slab in a controlledmanner so as to prevent fracturing of the slab by creating too large atemperature gradient between the interior and exterior portions of theslab. Further purposes of zone 34 are to cool the slab to a sufficientlyrigid condition that only periodic support e.g., driven stainless steelrolls, need be provided for transport of the slab, and to cool the slabto a point where it will no longer require a parting agent forseparation from other solid surfaces.

Before the slab leaves the collecting belt, it is cooled to a surfacetemperature of not less than 25 C. below the initial melting temperatureof the composition. This cooling generally takes place over a period offrom about to about 60 minutes and is generally dependent upon thethickness of the slab.

The temperature of the foamed ceramic slab as it exits from thecollection zone into the controlled cooling zone is from about 800 C. toabout 1800 C. In the controlled cooling zone, the slab is cooled to fromabout 700 C. to about l,200 C. over a period of from about to about 60minutes. The subsequent annealing process comprises cooling the materialto a temperature of about 200 to about 500 C. over a period of fromabout 2 to 8 hours. Below about 200 C. the ceramic slab is machined tothe desired size and shape by the use of tools suitable for cuttingordinary masonry.

Before machining, the ceramic slab has a cross-sectional area resemblingFIG. 2. The slab is machined to provide a rectangular cross-sectionalarea. With reference to FIG. 3, the machined slab (200) is essentiallyplanar in shape with a first major side (202) and a second major side(not shown) which is substantially parallel with the first major side.From FIG. 3, it can be seen that the slab contains a plurality of cells201 which are generally uniformly distributed throughout the slab. Atleast about 90 percent of the cells are less than 2 mm. in size. Whilethe thickness of the slab (i.e., the distance between the first andsecond major sides) varies with belt speed, rate of feed, bloatabilityof the composition, and other factors, the slab thickness withfrequently be about 9 inches or less.

The ceramic materials of the present invention are employed forstructural or insulating purposes after machining. For specific uses,the ceramic material can also be reinforced with rods, or varioussurface coatings can be applied, as where the material is to be used fordecorative purposes.

Suitable bloatable ceramic compositions comprise at least 45 percentsilica (SiO and not more than about 18 percent of fluxing oxides.Suitable fluxing oxides are, for example, FeO, CaO, MgO, M1 0, and X 0.The material should also contain from about 0.05 to 2.0 weight percentof an organic carbon source material wherein the carbon is not presentas carbonate. Suitable sources of organic carbon are, for example,organic carbon naturally present in clays, rice hulls, lampblack, coal,charcoal, oil shale and flour. In determining the amount of organiccarbon present in the bloatable ceramic composition, the weight ofoxygen, hydrogen, sulfur and other elements in the carbonaceous materialare not considered, i.e., organic carbon is simply the weight oforganically combined carbon present. The organic carbon content of aceramic composition is equivalent to the difi'erence between totalcarbon and inorganic carbon (i.e., carbon in carbonates). The methodemployed for determining total carbon is described by R. C. Rittner andR. Culmo, A Rapid Multiple Microdetermination of Carbon and HydrogenMicrochemica Acta, pages 631-640 (1964). The method for determininginorganic carbon is a detemiination of carbon present as carbonate. Themethod employed is that described by I-Iillebrand et al., at pages768-770 of Applied Inorganic Analysis, 2nd Edition, published by JohnWiley and Sons, Inc., of New York.

Generally, the initial melting temperature of particles of the bloatableceramic composition is from about 925 to 1500 C although materials withinitial melting temperatures above l,500 C. such as kaolin clay aresuitable if equipment withstanding such temperatures is available.

It has been found that the above-described requirements of fluxingoxides and silica are satisfied by argillaceous materials containing notless than 50 weight percent of clay minerals such as kaolinite, dickite,halloysite, illite, attapulgite and montmorillonite. Mixtures of clayminerals can also be employed. Other materials which can be employed incombination with the clay minerals include calcareous oil shale,dolomite, limestone, talc, feldspar, wollastonite, nepheline syenite andsimilar minerals. All such materials must be in a finely divided state,preferably 100 percent minus 200 mesh, with percent minus 325 mesh, orfiner.

In preparing the particles to be dropped, if more than one ingredient isrequired, the selected ingredients are intimately admixed to a uniformcomposition. The composition is moistened sufficiently so that itassumes the consistency of a plastic mass which is then molded toprovide particles having a maximum dimension within the range of onethirty-second to one-fourth inch. For example, the plastic mass can bemolded into suitably shaped particles by extruding it through a gratingto form strands. The strands are dried and broken into small particlesof an approximately uniform size such as one-sixteenth to one-eighthinch long and about one thirty-second inch in diameter. The resultingparticles are then thoroughly dried at a temperature of about 200 C. orslightly less, to remove substantially all water except chemicallycombined water. Suitable methods of forming particulate material aredescribed in U. S. Patents Nos. 3,259,171; 3,202,746 and 3,071,357. Thebulk density of the dried particles will generally be from about 0.9 toL1 gms/cmf, with the exact bulk density depending upon the compositionof the particles.

Where the iron content of the composition (expressed as Fe O is inexcess of about 4 percent by weight, it is necessary that the atmospherein the furnace chamber be a reducing atmosphere, i.e., during droppingand collection of the particles the atmosphere in the furnace chambermust be essentially oxygen-free to prevent oxidation of ferrous ironvalues to ferric oxide. The basic problem arises during the collectionand coalescence of the foam in the collection zone because the oxidationtakes place progressively from the surface and since the ferric iron isa poorer flux than the ferrous iron, the oxidized outer layers are morerefractory and will not coalesce at the same temperature as the interiorwhich tends to have the iron in the ferrous state. Thus, control of thecollection and coalescence steps are lost if a reducing or at leastnon-oxidizing atmosphere is not maintained. In the practice of theinvention, the desired reducing conditions are maintained if theatmosphere of the furnace chamber is essentially isolated from theoutside atmosphere, and if, in addition, the fuel gas mixture fed toburners 22, 29, and 28 contains less than the stoichiometric amount ofoxygen required to completely react with the fuel. For example, if thefuel is CH.,, the fuel gas mixture should consist volumetrically of notmore than 91 per cent air, with the balance being the fuel gas.

Within the limits of the process as described above, the density of thefoamed ceramic material can be controlled by varying the amount oforganic carbon in the bloatable ceramic composition. For example, for afoamed ceramic density of less than about 12 p.c.f., the organic carboncontent should be from about 0.50 to about 2.0 weight percent of thecomposition. For foamed ceramic densities of from 12 to 30 p.c.f., theorganic carbon content is from about 0.20 to about 0.50 weight per cent.For densities in excess of about 30 p.c.f., the organic carbon contentis less than about 0.20 weight percent.

The foamed ceramic product of the present invention is a rigid vitreousstructural element, also possessing excellent properties as aninsulating material. The product is resistant to acidic and basic mediaand is water insoluble. The ceramic product comprises a plurality ofclosed individual, non-communicating hollow cells which are uniformlydisposed throughout the product.

The ceramic foam material has a cross-sectionally uniform density. Formaterials with a density of from 6 to 12 p.c.f., thermal conductivity isfrom about 0.36 to about 0.70 BTU/ft F./hour/inch (75 F. meantemperature). At temperatures for the ceramic material of from about C.to about 950 C., specific heat is from about 0.15 to about 0.30BTU/pound. Moisture absorption is from about 0.1 to about 0.5 percent byvolume. The ceramic material (of 6 to 12 p.c.f. density) may be furthercharacterized as having a water vapor transmission of from about 0.0 to0.5 perm-inch.

ln characterizing the foam ceramic product, thermal conductivity isdetermined by employing ASTM test C-l7763 as modified by C-2406l. Waterabsorption is determined by ASTM test C27253 as modified by C-240-6l.Water vapor transmission is determined by ASTM test C-355-64. Within thetemperature range of from 0 C. to 500 C., specific heats are determinedby differential scanning calorimetry using a Perkin-Elmer, Model DSC-lBdifferential scanning calorimeter. At temperatures of from 500 C. to 950C., specific heats are determined by drop calorimetry. This technique isdescribed in the 1968 edition of McCullock and Scott, ExperimentalThermodynamics, Volume 1, Chapter 8.

The foamed ceramic products of the invention can be produced in avariety of colors. For example, where the bloatable raw materialemployed contains ferrous or ferric iron, and the foam is produced in areducing atmosphere (as described above), the foamed ceramic productwill be black or gray. A reddish color can be obtained by exposing theceramic product to an oxidizing atmosphere while the temperature of thefoam is in excess of about 750 C. Such exposure causes formation offerric oxide on the surface of the foam. if desired, a white ceramicfoam can be produced by employing, as the raw material, a kaolin claywhich is essentially free of color forming impurities such as the oxidesof iron, chromium, cobalt, vanadium, and nickel.

Preferred Embodiment A preferred embodiment of the invention is theprocess whereby foamed ceramic materials having a density of less thanabout 10 p.c.f. are obtained. These low density products are alsocharacterized as having substantially uniformly sized cells with anaverage diameter of less than about 2 mm. Generally, the averagediameter is less than about 1 mm.

It has been discovered that, using a continuous dropping process asdescribed above, only ceramic compositions wherein the particles have amaximum bloating capacity of less than about 25 p.c.f. will producefoamed ceramic material having a density of less than about 12 p.c.f. Inthe preferred embodiment of the invention, it is necessary that theparticles have a maximum bloating capacity of less than p.c.f. if foamedceramic materials of about 10 p.c.f. or less are to be produced.

The propensity of a ceramic composition to bloat to a density of lessthan p.c.f., or less than 20 p.c.f., is determined by fabricatingparticles of the composition, as described above, and passing theseparticles downwardly through a heated zone so that substantially all theparticles are bloated. The bloated particles are than cooled beforecollection so that they will not adhere to each other during collection.A sample of the bloated particles is then obtained.

The bulk density of the sample of bloated particles is determined byplacing the sample of bloated particles in a 25 milliliter graduatecylinder. The cylinder is dropped, base down, a distance of about oneinch onto a wooden table top. The cylinder is dropped twenty times. Thevolume of the particles is then noted and the weight of the particles isobtained. The bulk density of the particles is the weight in gramsdivided by the volume in cubic centimeters. Multiplying the abovequotient by 62.4 will yield the bulk density in pounds per cubic foot.The bulk density of dried non-bloated particles which have not beendropped through a heated zone is also determined by the method justdescribed.

In producing low density foamed ceramics other factors, in addition toproper choice of the bloatable ceramic composition, are to becontrolled. The temperature in the dropping zone should be regulated sothat immediately before collection, the ratio of the bulk density of thebloated particles to the bulk, density of the particles before droppingand bloating will fall within the range of from about 0.20 to about0.75. Preferably, the bulk density ratio will be from about 0.30 toabout 0.50. Where bulk density ratios within the above ranges areemployed, the particles have generally bloated to from about 50 topercent of their capacity, e.g., if the density of the particles beforebloating is from about 55 to about 70 p.c.f., the density of theparticles as collected will be from about 15 to about 50 p.c.f. Thetemperature of the collection zone and time of residence of theagglomerated ceramic material therein is regulated so that the particlesbloat to their maximum capacity and coalesce. Once coalescence hasoccurred, the ceramic material should be removed from the collectionzone into a controlled cooling or annealing zone as further heating inthe collection zone may result in enlargement of the cells andundesirable increases in density.

Particularly suitable ceramic compositions are those having theabove-described bloating propensities, and having, in addition thereto,an initial melting temperature, as previously defined, of from about 925C. to about 1,500 C. When such compositions are employed, thetemperature in the dropping zone is from about 175 C. to about 400 C.above the initial melting temperature of the composition. The-collectionzone temperature is from about 50 C. to about 250 C. above the initialmelting temperature of the composition. When the above temperatureranges are employed, the height of the dropping zone is generally fromabout 15 to about 25 feet, and the diameter of extended pellets rangesfrom about 0.010 inch to about 0.050 inch with the pelletlength/diameter ratio being from 1 to 10. The time of residence in thecollection zone is from about 0.75 to about 5 minutes.

Preferably the temperature in the dropping zone is from about 200 C. toabout 305 C. above the initial melting temperature of the composition,and the collection zone temperature is from about C. to about 225 C.above the initial melting temperature of the composition.

The ceramic material produced in the preferred embodiment of theinvention (described above) has a density of from about 6 to about 10p.c.f. Most frequently the ceramic material is produced in densities offrom about 6.8 to about 9 p.c.f. The material has a thermal conductivityof from about 0.36 to about 0.60 BTU/ft F./hour/inch, a specific heat offrom about 0.15 to about 0.30 BTU/pound (as measured over a temperaturerange of from 0 C. to 950 C.), and a moisture absorption of from about0.1 to about 0.5 percent by volume. The average cross-sectional diameterof the cells is less than 2.0 mm. Compressive strength of the materialis from about 20 to about 300 pounds per square inch, and flexurestrength is from 40 to 200 pounds per inch squared (as determined by 3point loading).

The following example is set forth to illustrate a preferred embodimentof the invention.

Example of the Invention Suitable clay was obtained. The clay (as mined)was bluegray in color and was of the general type of clays denominatedas Minford silts, as described in the Bulletin of the Geological Societyof America, Volume 42, pages 663-672, September, 1931.

Chemical analysis of the clay expressed as the oxides is as follows:

The clay contained about 0.57 weight per cent of organic carbon.Therefore, addition of organic carbon to the clay was not necessary.

As mined, the clay contained about 40 percent by wt water on a drybasis. The clay was extruded through a 0.020 inch grating and theresulting strands were dried. The dried strands were broken to yieldsmall particles with an approximately uniform size of from 0.020 to 0.20inch in length and about 0.020 inch in diameter. The particles were thendried at about 200 C. to remove all water except chemically combinedwater. The amount of chemically combined water in the pellets was about5.4 percent by weight. The bulk density of the pellets, as determined bythe method described above, was about 59 p.c.f.

The initial melting temperature of the pellets was about l,020 C.

The dried pellets were placed in a hopper at the top of a furnacesimilar to that depicted in FIG. 1. The particles exited from the hopperonto a grooved drum. The particles were subsequently dislodged from thegrooves of the drum by the force of gravity and passed through thefurnace chamber and were collected on a conveyor belt previously coatedwith parting agent. In the furnace chamber, the distance from the bottomof the drum to the collection belt was about 23 feet. The falling timefor the particles was about 0.8 second. About 2 feet directly below thebottom-most portion of the drum, the temperature in the furnace chamberwas about 1,260 C. The temperature of the surface of the foam measuredoptically in the collection zone was 1,190 1,2l C.

The fuel mixture fed to the burners consisted of about 90 percent byvolume of air with the balance being methane. As in FIG. 1, the furnacechamber and collection zone were essentially closed to the atmosphere.

As the particles passed downwardly through the furnace, samples wereobtained (from a point immediately above the collection zone) byinserting an air cooled stainless steel collection cup into the furnace.As collected, the particles were partially bloated and had a bulkdensity of about 21.5 p.c.f. The ratio of the bulk density of collectedparticles to the bulk density of bloatable ceramic particles in thehopper was about 0.36.

After passing the dropping zone, the partially bloated particles werecollected in a mass on a conveyor belt to form a slab of cellularagglomerated material. The speed of the belt was about 4 inches/minute.The temperature in the region immediately above the belt where theparticles were collected was about 1,210 C. The slab had an averagethickness of about 5 inches. The average residence time of particles(and ceramic material produced therefrom) in the collection zone wasabout 120 seconds.

As the slab passed into the controlled cooling zone, the surfacetemperature of the slab (as measured by optical pyrometer) was about1,200 C. The slab remained in the controlled cooling zone for about 50minutes. At a temperature of about 815 C., the slab was machined (bycutting), into 8 foot sections by a rotary saw. As the slab passed fromthe cooling zone into the annealer, the slab surface temperature wasabout 800 C.

In the annealer, the slab was cooled to a temperature of about 250 C.,over a period of about 5 hours. After removal from the annealer thesides, top and bottom of the slab were also machined so that the slabhad a rectangular cross-sectional area measuring about 4 inches by 16inches.

The cell size of the resulting foamed ceramic material had an averagecross-sectional diameter per cell less than 2 mm. The cells were closedor non-communicating so that air or water could not penetrate throughthe slab.

Slab density was about 7.0 to 8.0 pounds per foot, and flexure strengthwas from 65 to 75 pounds per inch. Thermal conductivity was about 0.37to 0.40 BTU/Ft! F./l-lr./in. Specific heat was about 0.175 Britishthermal units per pound at 25 C.; tensile strength was about 35 psi; andmoisture absorption was about 0.5 per cent by volume. The material wasresistant to both acidic and basic media, i.e., degradation did notoccur upon contact with these media.

What is claimed is:

1. A process for preparing an agglomerated foamed ceramic materialhaving a uniform density of about 12 pounds per cubic foot or lesscomprising:

a. passing particles of a bloatable ceramic composition downwardlythrough a heated dropping zone wherein the temperature is in excess ofthe initial melting temperature of the composition and is sufficient tobloat the particles to within about 50 to about percent of the maximumbloating capacity of the particles, by the time they reach the bottom ofthe dropping zone to produce in each particle a plurality of hollowcells,

b. collecting the bloated particles below the dropping zone in acollection zone thereby to form an initial agglomerate of a partiallyfoamed ceramic material,

c. heating said initial agglomerate at a temperature for a timesufficient to further bloat and coalesce the particles to form a foamedceramic slab, and

d. cooling the foamed ceramic slab.

2. A process as in claim 1 wherein the bloatable ceramic compositioncomprises at least about 3 weight percent of iron expressed as Fe o andwherein the atmosphere in the heating zones through which the bloatableceramic particles and agglomerated cellular material are passed is areducing atmosphere.

3. A process as in claim 1 wherein the bloatable ceramic composition hasa maximum bloating capacity of less than 25 p.c.f. and wherein thetemperature in the dropping zone is regulated so that the ratio of thebulk density of bloated particles immediately before collection thereof,to the bulk density of particles before dropping is from about 0.20 toabout 0.75.

4. The process of claim 3 wherein the bulk density ratio is from about0.30 to about 0.50.

5. A process as in claim 3 wherein the temperature of the dropping zoneis from about 175 C. to about 400 C. above the initial meltingtemperature of the composition, and wherein the temperature of thecollection zone is from about 50 C. to about 250 C. above the initialmelting temperature of the bloatable ceramic composition.

6. A process as in claim 5 wherein the temperature of the dropping zoneis from about 200 C. to about 305 C. above the initial meltingtemperature of the composition, and wherein the temperature of thecollection zone is from about C. to about 225 C. above the initialmelting temperature of the bloatable ceramic composition with the timeof residence of the collected particles and foamed ceramic slab in thepollection zone being about 0.5 to 5.0 minutes.

7. A process as in claim 6 wherein the initial melting temperature ofthe bloatable ceramic composition is about 1,020 C., the dropping zonetemperature is about 1,260 C., with the falling time for particles beingabout 0.80 second, and wherein the collection zone temperature is about1,165 C.

8. A process for preparing an agglomerated foamed ceramic materialhaving a uniform density of about 12 pounds per cubic foot or lesscomprising:

material having a uniform density of about 10 pounds per cubic foot orless comprising:

a. dropping particles of a bloatable ceramic composition a. droppingparticles of a bloatable ceramic composition containing from about 0.50to about 2.0 per cent by containing from about 0.50 to about 2.0 percentby 5 weight of organic carbon and having a maximum bloating weight ofOrganic carbon and having a maximum bloatmg capacity of less than about20 p.c.f. downwardly through p y of less than about 25 P- downwardlythrough a heated dropping zone wherein the temperature is sulfia heateddropping zone wherein the temperature is sufficiently in excess f theinitial melting temperature f the ciently in excess of the initialmelting temperature of the composition to bloat the particles Sumcienflyto provide a composition to bloat the particles sufficiently to providea ratio of tm bulk density of the bloated particles w of t t theparticles before dropping from about 0.20 to about g be ore topping mmabou o 8 out 0.75,

b. agglomerating the bloated particles below the dropping z i g g ggigggifiggg igi iz gf: i gxg g zone in a heated collection zone to form aninitial aglomerate ofa ania foamed ceramic material glomerate of apartially foamed ceramic material, 8 y

. c. heating said initial agglomerate at a temperature above c. heatingsaid initial agglomerate at a temperature above the inltial meltingtemperature of said composition and the initial melting temperature andbelow the temperab 1 th f h d f ture of the dropping zone for a timesufficient to further e e tempera; o t Z rogpmg g time bloat andcoalesce the particles to form afoamed ceramic sufficlcm to further oatan co esce t e as to agglomerate, and form a foamed ceramic agglomerate,and

d. cooling the foamed ceramic agglomerate. coolmg the foamed ceramicagglomerate 9. A process for preparing an agglomerated foamed ceramic

2. A process as in claim 1 wherein the bloatable ceramic compositioncomprises at least about 3 weight percent of iron expressed as Fe2O3 andwherein the atmosphere in the heating zones through which the bloatableceramic particles and agglomerated cellular material are passed is areducing atmosphere.
 3. A process as in claim 1 wherein the bloatableceramic composition has a maximum bloating capacity of less than 25p.c.f. and wherein the temperature in the dropping zone is regulated sothat the ratio of the bulk density of bloated particles immediatelybefore collection thereof, to the bulk density of particles beforedropping is from aBout 0.20 to about 0.75.
 4. The process of claim 3wherein the bulk density ratio is from about 0.30 to about 0.50.
 5. Aprocess as in claim 3 wherein the temperature of the dropping zone isfrom about 175* C. to about 400* C. above the initial meltingtemperature of the composition, and wherein the temperature of thecollection zone is from about 50* C. to about 250* C. above the initialmelting temperature of the bloatable ceramic composition.
 6. A processas in claim 5 wherein the temperature of the dropping zone is from about200* C. to about 305* C. above the initial melting temperature of thecomposition, and wherein the temperature of the collection zone is fromabout 125* C. to about 225* C. above the initial melting temperature ofthe bloatable ceramic composition with the time of residence of thecollected particles and foamed ceramic slab in the collection zone beingabout 0.5 to 5.0 minutes.
 7. A process as in claim 6 wherein the initialmelting temperature of the bloatable ceramic composition is about 1,020*C., the dropping zone temperature is about 1,260* C., with the fallingtime for particles being about 0.80 second, and wherein the collectionzone temperature is about 1,165* C.
 8. A process for preparing anagglomerated foamed ceramic material having a uniform density of about12 pounds per cubic foot or less comprising: a. dropping particles of abloatable ceramic composition containing from about 0.50 to about 2.0percent by weight of organic carbon and having a maximum bloatingcapacity of less than about 25 p.c.f. downwardly through a heateddropping zone wherein the temperature is sufficiently in excess of theinitial melting temperature of the composition to bloat the particlessufficiently to provide a ratio of the bulk density of the bloatedparticles immediately before collection thereof to the bulk density ofthe particles before dropping from about 0.20 to about 0.75, b.agglomerating the bloated particles below the dropping zone in a heatedcollection zone to form an initial agglomerate of a partially foamedceramic material, c. heating said initial agglomerate at a temperatureabove the initial melting temperature and below the temperature of thedropping zone for a time sufficient to further bloat and coalesce theparticles to form a foamed ceramic agglomerate, and d. cooling thefoamed ceramic agglomerate.
 9. A process for preparing an agglomeratedfoamed ceramic material having a uniform density of about 10 pounds percubic foot or less comprising: a. dropping particles of a bloatableceramic composition containing from about 0.50 to about 2.0 per cent byweight of organic carbon and having a maximum bloating capacity of lessthan about 20 p.c.f. downwardly through a heated dropping zone whereinthe temperature is sufficiently in excess of the initial meltingtemperature of the composition to bloat the particles sufficiently toprovide a ratio of the bulk density of the bloated particles immediatelybefore collection thereof to the bulk density of the particles beforedropping from about 0.30 to about 0.50, b. agglomerating the bloatedparticles below the dropping zone in a heated collection zone to form aninitial agglomerate of a partially foamed ceramic material, c. heatingsaid initial agglomerate at a temperature above the initial meltingtemperature of said composition and below the temperature of thedropping zone for a time sufficient to further bloat and coalesce theparticles to form a foamed ceramic agglomerate, and d. cooling thefoamed ceramic agglomerate.